Silyl-Osmium(IV)-Trihydride Complexes Stabilized by a Pincer Ether-Diphosphine: Formation and Reactions with Alkynes

Complex OsH4{κ3-P,O,P-[xant(PiPr2)2]} (1) activates the Si–H bond of triethylsilane, triphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane to give the silyl-osmium(IV)-trihydride derivatives OsH3(SiR3){κ3-P,O,P-[xant(PiPr2)2]} [SiR3 = SiEt3 (2), SiPh3 (3), SiMe(OSiMe3)2 (4)] and H2. The activation takes place via an unsaturated tetrahydride intermediate, resulting from the dissociation of the oxygen atom of the pincer ligand 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(PiPr2)2). This intermediate, which has been trapped to form OsH4{κ2-P,P-[xant(PiPr2)2]}(PiPr3) (5), coordinates the Si–H bond of the silanes to subsequently undergo a homolytic cleavage. Kinetics of the reaction along with the observed primary isotope effect demonstrates that the Si–H rupture is the rate-determining step of the activation. Complex 2 reacts with 1,1-diphenyl-2-propyn-1-ol and 1-phenyl-1-propyne. The reaction with the former affords Os{C≡CC(OH)Ph2}2{=C=CHC(OH)Ph2}{κ3-P,O,P-[xant(PiPr2)2]} (6), which catalyzes the conversion of the propargylic alcohol into (E)-2-(5,5-diphenylfuran-2(5H)-ylidene)-1,1-diphenylethan-1-ol, via (Z)-enynediol. In methanol, the hydroxyvinylidene ligand of 6 dehydrates to allenylidene, generating Os{C≡CC(OH)Ph2}2{=C=C=CPh2}{κ3-P,O,P-[xant(PiPr2)2]} (7). The reaction of 2 with 1-phenyl-1-propyne leads to OsH{κ1-C,η2-[C6H4CH2CH=CH2]}{κ3-P,O,P-[xant(PiPr2)2]} (8) and PhCH2CH=CH(SiEt3).


■ INTRODUCTION
Polyhydride complexes, L n MH x (x ≥ 3), are transition metal species bearing enough hydrogen atoms bound to the metal center to form both classical hydride and dihydrogen ligands. A noticeable chemical characteristic of the platinum group metal complexes of this class is their proven ability to promote σbond activation reactions. 1 Several features of the H-donor ligands explain the ability of such complexes to break σ-bonds. Classical hydrides behave as Brønsted bases facilitating the heterolytic split, whereas dihydrogen ligands display the tendency to be released from the metal center, generating highly unsaturated species that promote the homolytic cleavage. The activation produces an increase in the coordination number of the metal center of a coordinatively congested species, and in some cases, the oxidation number of an ion already highly oxidized. Consequently, activations involving E−H bonds are favored since the initial coordination number and oxidation of the metal center are rapidly restored after the activation, by removal of molecular hydrogen. Among the activated E−H bonds, the C−H bonds are predominant. 1,2 On the other hand, the Si−H bond activation has received scarce attention, 3 although some silyl-metal-polyhydride derivatives are known mainly for osmium 4 and iridium. 5 Like the metal-mediated C−H bond cleavage, the Si−H bond activation takes place via σ-intermediates, where the Si−H bond coordinates to the metal center. Nevertheless, a greater variety in the strength degrees has been described for the interactions M−HSi than for the M−HC ones. 6 The Si−H bond activation reactions are noticeable because of the relevance of the M−SiR 3 derivatives as intermediate species in the synthesis of chlorosilanes, 7 the SiH/OH coupling, 8 and the hydrosilylation of unsaturated organic substrates, 9 including alkynes. 10 Alkynes are fundamental molecules in organic synthesis, 11 which display great relevance in organometallics due to their use as precursors of different functional groups 12 and as building blocks in the formation of new ligands and interesting metallacycles. 13 Their reactions with transition metal hydride complexes allow for the generation of single, double, and triple M−C bonds, depending on the nature of the metal center, the ligands, and the substituents of the alkyne. 12 Increasing the number of hydride ligands of the complex facilitates the use of alkyne building blocks since it permits to increase the number of such molecules accessing into the metal center, as a consequence of the increment of the number of possible reactions. The presence of a higher number of organic fragments attached to the metal favors a higher variety of C−C coupling reactions. 14 Polyhydride complexes have the ability of activating the C(sp)−H bond of terminal alkynes, whereas they hydrogenate the C−C triple bond of the disubstituted ones. The activation usually leads to alkynyl species, which are able to act as efficient catalyst precursors in the dimerization of such substrates to afford both enynes and butatrienes; 15 skeletons are of great interest because they are present in some biologically active natural products and in synthetic intermediates for the preparation of highly substituted aromatic rings and by their connection with materials science. 16 Terminal propargylic alcohols, HC� CCR 1 R 2 (OH), are alcohol-functionalized alkynes widely employed in organic synthesis as multifunctional reagents 17 and in organometallics as allenylidene ligand precursors. 18 Although they are noticeable molecules by both reasons, their reactions with polyhydride complexes have not been studied.
Pincer ligands are having a dramatic impact in the modern coordination chemistry due to their ability to stabilize less common coordination polyhedra, which favor unusual oxidation states of the metal. 19 Polyhydride chemistry is not alien to this influence, 19a although such ligands have been comparatively much less employed than in other areas, probably because pincer ligands saturate three coordination positions, reducing the number of sites for the hydrides. In this context, hemilabile pincer ligands are of great interest. A particular class with this characteristic is the P,O,Pdiphosphines. 20 Among them, 9,9-dimethyl-4,5-bis-(diisopropylphosphino)xanthene (xant(P i Pr 2 ) 2 ) occupies a prominent place due of its coordinating flexibility. 21 Although the κ 3 -P,O,P-mer mode is its most usual coordination, 8d,21c,22 complexes bearing diphosphine κ 3 -P,O,P-fac, 21c,23 κ 2 -P,P-cis, 24 and κ 2 -P,P-trans 21c,25 are also known. Transformations involving (xant(P i Pr 2 ) 2 )-M derivatives suggest that this etherdiphosphine changes its disposition at the metal coordination sphere to be adapted to the thermodynamic needs of the reactions. As a result, a wide variety of ruthenium, 22b,e osmium, 22b,e,f,j,23a,b,24a,c rhodium, 8d,21c,22a,c,d,g,h,k,l,n,24b,d and iridium 8d,22a,b,i,m,24e complexes stabilized by this diphosphine have been reported in the past years, which undergo fascinating transformations and promote interesting reactions, including catalytic processes. 8d,22b−d,h,k−m,24a,b,d,e,26 Accordingly, in 2013, we reported that sodium hydride in tetrahydrofuran removes the chloride ligands of OsCl 2 {κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}{κ 1 -S-[DMSO]} to give the hexahydride derivative OsH 6 {κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}, under 3 atm of hydrogen, at 50°C. This κ 2 -P,P-trans-diphosphine-osmium-(VI)-polyhydride slowly loses a hydrogen molecule in methanol, under an argon atmosphere, at temperatures higher than 293 K. The coordination of the oxygen atom to the metal center stabilizes the tetrahydride derivative OsH 4 {κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}. In contrast to its precursor, this tetrahydride bears a κ 3 -P,O,P-mer-diphosphine (Scheme 1). 24a The handy availability of the tetrahydride complex along with the hemilability of diphosphine, also proven in the hydride chemistry, as shown in Scheme 1, prompted us to investigate the tetrahydride-promoted Si−H bond activation of silanes. We searched for a silyl-osmium-polyhydride system, allowing us to study the reactivity of such class of complexes with alkynes, including alkynols. This paper describes such activation, including its mechanism, and the reactions of the resulting pohyhydride species with 1,1-diphenyl-2-propyn-1-ol and 1-phenyl-1-propyne.
The presence of a silyl ligand in complexes 2−4 is proved by the X-ray structure of the triphenylsilyl derivative 3. Figure 1 shows the molecular diagram. The polyhedron around the metal center can be idealized as a pentagonal bipyramid. Etherdiphosphine, which is coordinated in the mer-fashion, disposes the P i Pr 2 arms at apical positions, forming a P−Os−P angle of 158.63(4)°. The base is defined by the oxygen atom, the silyl Scheme 1. Synthesis of OsH 4 {κ 3 -P,O,P-[xant(P i   group, and the hydride ligands. The oxygen atom is situated between H(01) and H(02), whereas the silyl group lies between H(01) and H(03). The classical-polyhydride nature of these compounds is supported by the separations between the hydrides H(02) and H(03) of 1.71(4) Å, obtained from the X-ray diffraction analysis, and 1.826 Å, calculated for the DFT-optimized structure. Both the X-ray structure and that DFT calculated reveal relatively short separations between the silicon atom and the hydrides H(01) and H(03) of 2.16(3) and 2.21(3) Å and 2.241 and 2.230 Å, respectively, which could suggest the existence of the denoted "secondary interactions between silicon and hydrogen atoms (SISHA)" (1.9−2.4 Å). 3,6b Nevertheless, atoms in molecules (AIM) calculations do not show any bond path running between the involved atoms ( Figure S1). Thus, such values seem to be a consequence of the size of the atoms and their positions in the complex but not of the presence of any bonding interaction between them. In this context, we note that the boroncounterpart OsH 2 (η 2 -H-Bcat){κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (HBcat = catecholborane) is in contrast to the 2−4 a hydride-osmium(II)-(σ-borane) derivative. 22f Although there is a marked diagonal relationship between boron and silicon, 27 it seems that the stronger acidity of boron with regard to silicon favors the hydrogen-heteroatom interaction in this case.
Hydrides H(02) and H(03) undergo a thermally activated site exchange process in toluene-d 8 , which occurs with low activation energy (Table 1). Thus, at 313 K, the high-field region of the 1 H NMR spectra of the three polyhydrides shows two resonances in a 1:2 intensity ratio, at about −1 ppm and between −10.0 and −12.5 ppm. The first of them is due to H(01), and the second one corresponds to H(02) and H(03). Between 283 and 223 K, depending on the silyl ligand, decoalescence of the higher field signal takes place to afford two resonances. Accordingly, at temperatures lower than 223 K, the spectra display a resonance for each inequivalent hydride ligand, at the chemical shifts shown in Table 1. The 300 MHz T 1(min) values for the resonance assigned to the hydride ligands H(02) and H(03) of 2 and 3 were also determined at 248 and 230 K, respectively. The obtained values of 231 ± 3 (2) and 251 ± 3 (5) ms lead to separations of 1.76 (2) and 1.79 (3) Å between such hydrides, 28 which compare very nicely with those obtained from the X-ray diffraction analysis and DFT calculations for 3. In agreement with the structure shown in Figure 1, the 31 P{ 1 H} NMR spectra of 2−4 show a singlet between 54 and 47 ppm, as expected for the equivalent P i Pr 2 arms of diphosphine. A characteristic feature of these compounds is also the presence of a triplet ( 2 J Si−P ≈ 5 Hz) at about 2 ppm for 2 and 3 and at −19.8 ppm for 4, in the respective 29 Si{ 1 H} NMR spectrum.
Mechanism of the Si−H Bond Activation. Complex 1 is a coordinately saturated species. Thus, the Si−H bond activation of the silanes requires the previous generation of a coordination vacancy at the metal center. In principle, such a process could occur in two different manners (Scheme 3): by reductive elimination of molecular hydrogen (route a) and through the dissociation of the hemilabile ether function of diphosphine (route b). In the first case, the activation would take place via osmium(II) intermediates; the unsaturated dihydride a 1 should coordinate the Si−H bond of the silanes to afford the osmium(II) σ-intermediates a 2 , which could evolve into 2−4 by homolytic rupture of the coordinate σ-bond. In the second one, the activation involves intermediates of osmium(IV) and osmium(VI); similarly to a 1 , the unsaturated osmium(IV)-tetrahydride b 1 should coordinate the Si−H bond of the silanes to give the osmium(IV) σ-intermediates b 2 . These intermediates could lead to 2−4 by means of two different pathways associated to the type of activation undergone by the coordinated σ-bond. A hydride-promoted heterolytic rupture should directly give the silyl-osmium(IV)trihydride derivatives, whereas a homolytic activation followed by reductive elimination of molecular hydrogen would afford the silyl products via silyl-osmium(VI)-pentahydride intermediates b 3 . Complexes with two triisopropylphosphine ligands instead of ether-diphosphine resembling to b 3 have been recently isolated and fully characterized, including the Xray analysis structure of one of them in our laboratory. 29 We reasoned that the use of R 3 SiD instead of R 3 SiH should allow us to discern between routes a and b and establish the nature of the Si−H rupture. Indeed, route a should afford one deuterium atom at the hydride position, whereas a hydridepromoted heterolytic rupture through route b should lead to a totally protiated product. On the other hand, the homolytic activation to form intermediate b 3 would permit a deuterium amount intermediate between zero and one at the metal center since the position exchange processes, typical in this class of pentahydride complexes, should carry the deuterium atom to different positions, before the reductive elimination. Addition of 2.0 equiv of (Me 3 SiO) 2 MeSiD to a solution of 1 in toluened 8 , at 110°C leads to a partially deuterated 2-d 0.5 species containing 0.5 deuterium atoms distributed between the H(02) and H(03) positions ( Figure S49). As mentioned above, such a deuterium amount at the metal center supports the route b through intermediate b 3 as the preferred one for the Si−H bond activation promoted by 1.
To gain additional evidence in favor of the route b, we decided to trap the intermediate b 1 with a 2e-donor ligand such as triisopropylphosphine. In methanol, the stirring of 1 in the presence of 1.0 equiv of phosphine affords the expected tetrahydride OsH 4 {κ 2 -P,P-[xant(P i Pr 2 ) 2 ]}(P i Pr 3 ) (5 in Scheme 3), which was isolated as a white solid in 78% yield and characterized by X-ray diffraction analysis. Figure 2 gives a view of the structure. The disposition of donor atoms around the metal center can be described as a piano stool geometry of the ideal C s symmetry, where the four-membered face is formed by the phosphorus atoms of the chelating diphosphine and the hydrides H(01) and H(04), whereas the threemembered face is defined by the hydrides H(02) and H(03) and the phosphorus atom of triisopropylphosphine. Such ligand disposition is unusual for osmium(IV)-tetrahydride complexes of the class OsH 4 (PR 3 ) 3 , which generally displays a pentagonal bipyramid arrangement. 30 In solution of toluene-d 8 , the structure is not rigid in accordance with the similar stability of the usual four geometries of seven-coordinate complexes and the low activation energy for their interconversion. 31 Thus, the 1 H NMR spectrum at room temperature shows a broad resonance for the four hydride ligands at −10.87 ppm, which Organometallics pubs.acs.org/Organometallics Article splits into two broad signals, at −9.99 and −11.90 ppm, of 1:1 intensity ratio, at 235 K. The 31 P{ 1 H} spectrum is also temperature-dependent ( Figure S27). At room temperature, the spectrum shows at 44.6 ppm an apparent triplet due to monodentate phosphine and at 8.4 ppm a broad signal corresponding to diphosphine. At temperatures lower than 253 K, the triplet is transformed into a complex signal, whereas the broad resonance splits into two signals at 14.7 and 2.3 ppm. Once the pathway for the Si−H bond activation was established, we decided to confirm it and ascertain the ratedetermining step by means of the kinetics investigation of the reaction of 1 with Et 3 SiH. The transformation of 1 into 2, studied by 31     0.98 ( Figure S54), revealing that the activation is also firstorder in the silane concentration (a = 1 in eq 2), and therefore, the rate law is that given in eq 3. A plot of k obs versus [Et 3 SiH] ( Figure 4) provides a value of (1. The rate of the reaction of 1 with Et 3 SiD is significantly slower than that with Et 3 SiH. The ratio k H /k D gives a primary isotope effect of 4.2, which strongly supports the homolytic cleavage of the Si−H bond as the rate-determining step in the formation of the silyl-osmium(IV)-trihydride complexes. 32 According to the rate-determining step approximation, the formation of 2 can be also described by eq 4. [ Since the reductive elimination of molecular hydrogen from intermediate b 3 is the fast step of the silyl product formation, the concentration of the intermediate b 2 can be calculated as Intermediate b 1 is not spectroscopically detected. As a consequence, we can assume that K 1 + K 1 K 2 [Et 3 SiH] ≪ 1, and therefore, [b 2 ] can be described as Combining eqs 4 and 7, we obtain eq 8, which is additional evidence in favor of route b and reveals that k = k a K 1 K 2 , that is, the experimental rate constant k is proportional to the rate constant k a and the equilibrium constants K 1 and K 2 .
Complex 6 was characterized by X-ray diffraction analysis.  (3) and 1.316(4) Å, respectively, also compare well with those previously reported for complexes of this class and strongly support the presence of double bonds between the involved atoms. In benzene-d 6 , at room temperature, the vinylidene ligand rotates around the metalvinylidene axis, as is usual for this class of compounds. Thus, the 13 C{ 1 H} NMR spectrum reveals the presence of only one type of hydroxyalkynyl ligand; the atoms C α and C β of the triple bond give rise to a triplet ( 2 J C−P = 11.8 Hz) at 102.0 ppm and a singlet at 75.5 ppm, respectively, while the C α and C β atoms of the vinylidene ligand generate two triplets, at 291.6 ( 2 J C−P = 9.1 Hz) and 111.5 ( 3 J C−P = 3.8 Hz) ppm, respectively. In the 1 H NMR spectrum, the most noticeable resonance is that due to the C β H-hydrogen atom of the vinylidene moiety, which appears at 2.45 ppm as a triplet ( 4 J H−P = 2.7 Hz). The 31 P{ 1 H} NMR spectrum displays a singlet at 12.7 ppm, as expected for equivalent P i Pr 2 arms.
Complex 7 was also characterized by X-ray diffraction analysis. Figure 6 shows a molecular diagram of the species. The geometry around the metal center resembles that of 6, with the allenylidene group in the position of the hydroxyvinylidene ligand. C 3 -cumulene binds to the osmium atom in a nearly linear fashion [Os−C(1)−C(2) = 178.7(3)°a nd C (1) (3)] compare well with those previously reported for structurally characterized complexes of this class. In agreement with them, the C(1)− C(2) and C(2)−C (3) values suggest a notable contribution of the canonical form [M] − −C�C−C + Ph 2 to the structure of cumulene. 22b,23b,36 The C 3 -chain gives rise to three triplets in the 13 C{ 1 H} NMR spectrum, which in benzene-d 6 appear at 251.3 (C α , 2 J C−P = 10.1 Hz), 247.3 (C β , 3 J C−P = 3.7 Hz), and 154.2 (C γ , 4 J C−P = 2.1 Hz) ppm. Noticeable resonances of this spectrum are also a triplet ( 2 J C−P = 11.7 Hz) at 103.8 ppm and a singlet at 75.1 ppm, respectively, corresponding to the C α and C β atoms of the triple bond of the hydroxyalkynyl ligands. In agreement with 6, the 31 P{ 1 H} spectrum displays a singlet at 13.3 ppm.
Scheme 5 summarizes a rare example of the tandemcatalyzed reaction, 37 which has no precedent in the osmium chemistry. The process involves the dimerization of the functionalized alkyne and the subsequent cycloisomerization of the resulting enynediol to afford an interesting furanylideneethanol. The formation of the five-membered heterocycle is noteworthy since such a moiety is present as a structural subunit in numerous natural products with a variety of applications in several fields. Metal catalysts for the dimerization of terminal propargylic alcohols are scarce. In contrast to 6, the majority of them lead to products resulting from a head-to-head (E)-coupling 38 and in some cases, from a head-to-tail dimerization. 39 The formation of alkylidenebenzocyclobutenyl alcohols has also been observed. 40 Catalysts for the cycloisomerization of enynols are even more rare, 41 and as far as we know, have not been applied to diols.
Complex 2 also reacts with internal alkynes such as 1phenyl-1-propyne. Treatment of its solutions in toluene with   Complex 8 was obtained as a yellow solid in 65% yield and characterized by X-ray diffraction analysis. Figure 7 shows a view of the molecule. The coordination around the osmium atom can be described as a very distorted octahedron. The distortion is consequence of the steric hindrance experienced by the P i Pr 2 arms of the mer-disposed ether-diphosphine and the olefinic C(8)−C(9) bond and explains why only one from the two olefins generated in the reaction selectively attaches to the osmium atom, the olefin with less steric hindrance in its C−C double bond. The parallel disposition of the olefinic C(8)−C(9) bond to the P(1)−Os−P(2) axis decreases the phosphine bite angle until 141.44(6)°, a value strongly deviated from the ideal 180°. In agreement with the concerted character of the oxidative addition of the C−H bond, the hydride ligand H(01) and the metalated carbon atom C(1) of the phenyl group are mutually cis-disposed. The oxygen atom of the diphosphine lies trans to the phenyl group [O(1)−Os− C(1) = 168.0(2)°], whereas the C(8)−C(9) bond situates trans to H(01). The C(8)−C(9) bond coordinates to the osmium atom with Os−C(8) and Os−C(9) distances of 2.173(6) and 2.178(6) Å, which are almost identical. The coordination causes a significant elongation of the double bond, as expected for the Chatt−Dewar−Ducanson bonding model. Thus, the C(8)−C(9) bond length of 1.424(9) Å is notably longer than those usually observed for free C−C double bonds of around 1.34 Å. 42 In accordance with the strong addition of this bond to the metal center, the resonances corresponding to C(8) and C(9) are observed at significant high fields, 41.0 and 36.3 ppm, respectively, in the 13 C{ 1 H} NMR spectrum in benzene-d 6 . In a consistent manner, the signals due to the associated hydrogen atoms appear at 3.67 [C(8)H], and 2.49 and 1.84 [C(9)H 2 ] ppm in the 1 H NMR spectrum, which displays the hydride resonance at −7.15 ppm as a doublet of doublets with H−P coupling constants of 33.0 and 37.4 Hz. As a result of the asymmetry imposed by the coordination of the olefin, the P i Pr 2 arms of the pincer are inequivalent. Accordingly, the 31 P{ 1 H} NMR spectrum shows an AB spin system centered at 36.2 ppm and defined by Δν = 726 Hz and J A−B = 164 Hz.

■ CONCLUDING REMARKS
This study has revealed that the tetrahydride OsH 4 {κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} activates the Si−H bond of tertiary silanes to form silyl-osmium(IV)-trihydride derivatives, in spite of its saturated character. A detailed study of the mechanism of the activation pointed out that the activation is possible because of the hemilabile nature of ether-diphosphine, which dissociates its oxygen atom to permit the Si−H coordination of the silane. The subsequent oxidative addition of the coordinated bond, followed by the reductive elimination of molecular hydrogen, affords the silylated polyhydrides. Kinetics of the addition and the observed primary isotope effect demonstrate that the rupture of the Si−H bond is the rate-determining step of the metal silylation.
■ EXPERIMENTAL SECTION General Information. All reactions were performed with rigorous exclusion of air and moisture at an argon/vacuum manifold using standard Schlenk-tube or glovebox techniques. Alkynes and silanes were purchased from commercial sources and distilled in a Kugelrohr distillation oven prior to use. Complex 1 was prepared according to the published method. 24a Instrumental methods, X-ray and theoretical calculation details, and NMR and IR spectra (Figures S2−S48)

Preparation of OsH 3 (SiPh 3 ){κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (3).
A solution of 1 (200 mg, 0.31 mmol) in toluene (5 mL) was treated with HSiPh 3 (164 mg, 0.62 mmol). The resulting mixture was heated at reflux for 16 h. After this time, the mixture was concentrated to dryness to afford a yellowish residue. Addition of methanol (5 mL) caused the precipitation of a white solid which was washed with additional methanol (3 × 5 mL) and dried in vacuo. Yield: 275 mg (97%). Colorless single crystals suitable for X-ray diffraction analysis were obtained from a saturated solution of 3 in benzene at room temperature. Anal. Calcd for C 45 H 58 OOsP 2 Si: C, 60.38; H, 6.53.